The invention relates to a gas turbine aeroengine fan casing, and more particularly to a fan casing made of composite material comprising fiber reinforcement densified by a matrix.
In a gas turbine aeroengine, the fan casing performs several functions. It provides aerodynamic continuity for the secondary stream through the engine, it supports an abradable material facing the tips of the fan blades, it supports a structure that is generally in the form of acoustic panels in order to attenuate soundwaves at the inlet of the engine, and it incorporates or supports a shield for retaining debris projected against the inside face of the casing, such as ingested foreign bodies or damaged blade debris.
Proposals have already been made to make a fan casing out of composite material. Document US 2008/206048, the content of which is incorporated herein by reference, describes a method comprising making a fiber preform for the casing by taking layers of a fiber texture obtained by three-dimensional weaving and presenting varying thickness, by winding the layers in superposition onto a former, and then by densifying the preform with a matrix, typically a polymer matrix. The use of a woven texture that presents varying thickness makes it possible to incorporate the retention shield merely by a localized increase in thickness.
Such a composite material fan casing can perform the functions of supporting abradable material and of supporting acoustic panels. Nevertheless, mounting pieces of equipment on the casing can give rise to difficulty, particularly when the pieces of equipment are heavy and/or subjected to particular forces. A typical example of such a piece of equipment is the accessory gearbox (AGB) that contains gearing mechanically coupled to one of the shafts of the turbine and that has various rotary members mounted thereon (such as starter-generators, pumps, alternators, . . . ). In particular for reasons of compactness, it may be necessary or desirable to mount the AGB on the fan casing. With a metal casing, that is typically achieved by fastening metal fittings on ribs positioned on the outside face of the casing, with the housing of the AGB being mounted on the fittings. That solution is difficult or impossible to transpose to a fan casing that is made of composite material because of the high risk of severely damaging the composite material under the effect of the weight of the AGB and the members that it carries, and under the effect of the forces to which the AGB may be subjected in the event of exceptional loads being applied to the structure of the engine, in particular in the event of a fan blade breaking.
One solution might consist in reducing the axial dimension, or length, of the casing that is made of composite material and in increasing correspondingly the length of the outer shroud of an intermediate metal casing that extends the fan casing from its downstream end, so as to make it possible to fasten the AGB mounting fittings on the intermediate casing. Nevertheless, the weight advantage of a fan casing that is made of a composite material compared with a fan casing that is made of metal would be significantly reduced thereby.
Another solution as proposed in document US 2011/0085897 consists in surrounding the fan casing made of a composite material with a collar that carries the equipment fastener parts. The collar is fastened to the outside surface of the casing by adhesive. The forces, in particular shear or traction forces, that are generated in service by the presence of the equipment are thus transmitted to the casing with a high risk of the composite material of the casing being damaged.